In this description, reference to steam-based thermal recovery operations or processes indicates that steam injection into a hydrocarbon reservoir is either an exclusive or a nonexclusive aspect of the injection portion of the process. When steam is a non-exclusive aspect of the recovery process, this implies that other substances may be co-injected or injected sequentially with the steam. Thus, by way of example, steam-based thermal recovery operations in which steam is a non-exclusive aspect of the injection stream can include such concurrent or sequential supplements to the injected steam as light liquid hydrocarbons, gaseous hydrocarbons such as natural gas, or non-hydrocarbon substances, such as nitrogen or air.
In steam-based thermal recovery operations that are typically aimed at recovering bitumen or heavy oil, a longstanding effective approach to raising steam has involved the use of once-through steam generators. Feedwater to the once-through steam generator (OTSG) can come from many sources and, depending upon the properties of the raw water, is treated to render it suitable as a feed stream for a OTSG. The steam thus generated is injected into an oil sand reservoir containing bitumen, or into a reservoir containing heavy oil. The steam heats and mobilizes the bitumen or heavy oil. When the mobile hydrocarbon liquid is lifted to the surface, it is part of a mixture that also contains water from condensed steam, formation water, and various minerals and other constituents which may be dissolved or suspended in the mixture, along with vapor and gaseous constituents.
After appropriate gas-liquid separation followed by treatment of the liquid stream to substantially segregate produced water from the produced liquid hydrocarbon constituent, current oilfield practice often involves some form of re-cycling of the produced water. This typically entails some form of treatment of the produced water that renders it suitable for re-use as boiler feedwater in the once-through steam generators. This treatment normally includes removal of hardness and reduction in silica levels.
It should be noted that a once-through steam generator is normally operated so that wet steam, typically around 80 percent quality, is generated, although other levels of steam quality may be selected. In some types of thermal recovery operation, the entire stream of wet steam is injected into the reservoir, for example Cyclic Steam Stimulation (CSS). In other types of thermal recovery operations, such as those involving Steam Assisted Gravity Drainage (SAGD), the wet steam is first separated into its vapor and liquid components by means of a steam separator at the outlet of the once-through steam generator. The vapor component exiting the steam separator, consisting of substantially 100 percent quality steam, also known as dry saturated steam, is injected into the reservoir. However, the liquid component, referred to as blowdown contains in concentrated form essentially all of the impurities that were originally in the feedwater.
The blowdown, with its high impurity levels, may be disposed of, often after some form of heat exchange, or may be re-routed back to the inlet of the water treatment facility where it is treated and re-used. Alternatively, the blowdown may be routed to some other appropriate point in the process that is upstream of the once-through steam generator.
Under current industry practice, re-cycling of blowdown by re-routing it from the outlet of the steam generator back to the inlet of the water treatment facility is often an acceptable approach. The more blowdown that can be utilized in this way, the less the need for make-up water from some higher quality source. However, a disadvantage of this approach is that the size of the water treatment facility has to be enlarged to accommodate the blowdown stream, and the operation has to be adjusted accordingly, thereby incurring additional capital and operating costs. Also, in some circumstances, the levels of Total Dissolved Solids in the blowdown stream limit the amount of blowdown that can be re-cycled to the water treatment facility.
Analogously, an evaporator may be employed upstream of the once-through steam generator when using produced water to generate steam. The blowdown from the once-through steam generator can be routed back to the evaporator inlet or feed tank for recycling through the evaporator. Evaporators are energy intensive and are therefore not always a desirable alternative. However, if one were to choose an evaporator for this service, the facilities would need to be sized and designed to accommodate the re-cycled stream. Also, as the evaporator operates at essentially atmospheric conditions, some irreversible energy loss would be incurred when the high pressure blowdown from the once-through steam generator is routed to the evaporator inlet or feed tank.
A further alternative involves treatment of the boiler blowdown. This treatment can include chemical means to reduce hardness and silica, or can involve physical means such as evaporation. However, this is a costly alternative.
It is, therefore, desirable to provide an improved method and apparatus for steam generation that provides improved handling of boiler blowdown.